From what I’ve seen of the 2015 domestic EBA, the base salaries are not that dissimilar for a level one 737 F/O and 1st year ATR Capt.
Not sure of the criteria to move up the levels on the jet, but a level 3 F/O on the 737 appears to have a base about 30% greater than an ATR captain of comparable years of service. Then there’s overtime and allowances.